This is standard for CRM customizations. There are two rules to consider: 1. Importing a customization for an entity (not isv.config or sitemap) does not destroy data. 2. Last import wins for form and view designs.

The consequence of rule 1 means that when you import an entity on top of an existing entity, the import process will only create new attributes and relationships and will NOT remove existing relationships or attributes that are already present. Bottom
line - you keep your schema and data.

You can exploit rule #2 by exporting your existing customizations, then import the portal customizations, then import the export you just made before you imported. The end result will be that you will be back with your existing forms and views, but
with the additional entities and attributes required for the portal accelerators. I would recommend this approach. Once you do this, all you then have to do is manually add the web authentication tab and required fields in the contact form.
So before you re-import your backup, simply open the contact form and take a screenshot of the fields on that form, and this screenshot will make it easier for you to add the form back in after you reimport your customizations.